I wrote a blog post about it:
The key language...
Why The Expense of Magic Items Makes Them Unviable as Goods
Given that magic items in ACKS are expensive to create, why aren't they valuable economic goods? The reason lies in the often-misunderstood differential between cost and value. Cost is what it requires to produce a good. Value is what someone is willing to pay for the good. As an example, let's say that I'm an attorney who earns $100 per hour. Let's say that instead of doing client work, I decided to spend 8 hours sketching a drawing of my office. Being an attorney and not an artist, my drawing is not very good. Yet its economic cost to produce was the loss of 8 hours of my time, e.g. (8 x $100) $800.
Like my hypothetical office sketch, magic items in ACKS cost more than their value. Consider the humble +1 sword, which requires 5,000gp and one month to create. In the hands of a hireling, the difference between wielding a sword, and wielding a +1 sword, is identical to the difference between being a normal man (Attack throw 11, damage by weapon) and being a 1st level fighter (attack throw 10, damage by weapon +1).
The cost to buy a 0th level slave-soldier in ACKS is 415gp. The cost to buy a 1st level slave soldier in ACKS is 830gp, for a net difference of 415gp. An overlord might be willing to pay 415gp for a sword +1, the difference between the cost of a 0th level slave-soldier and a 1st level slave-soldier, but he wouldn't be willing to pay 5000gp. For that price he could have 6 more 1st level slave-soldiers!
Why, then, might a sword +1 ever be created? Only a few reasons would justify its production:
1. Conspicuous consumption. A lord might commission a magic sword in order to show off his wealth. Such a sword would almost certainly be decorated lavishly and bear his house sigil and other markings.
2. Need to fight enchanted monsters. An adventurer facing, e.g., undead might need a magical weapon in order to harm them.
3. Performance at any price. A fighter of sufficient means, taking extreme personal risk, might be willing to pay virtually any price for an additional edge.
Now, consider the sword +1 extracted from a dungeon. It is of uncertain purview and ownership history. While finely decorated, it is unlikely to have precisely the decorations that a lord would seek to expend resources on. A lord might buy such an item as a curio -- "Ah, yes, Fritz, this is that blade that was found in the Ruins of Zahar! Charming work, what?" -- but he would pay much less than he'd pay for a commissioned item of his choice.
Other adventurers might buy the sword - if they don't have one of their own, and if they have spare cash, and if they trust their rivals enough to do business with them. This is a fairly small market, and an unefficient one. The likelihood of another adventurer of around the same level being in the same area and needing the specific item is low.
Finally, a mage's guild or other broker might buy the item at a steep discount, hoping to re-sell it later when there is an urgent need for it. Perhaps the mage's guild recognizes that the ancient sword belongs to a particular, distant noble house that would pay for it. They might buy it from the adventurers at a deep discount, absorb the cost of identification, authenticizing, and transport, and then sell it at a mark-up.
But in all of these cases, no one is buying the magic item because of the value of its bonus, per se.
The Industrial Revolution Will Not Be Magickified
It's worth remembering that the ancient engineer Hero of Alexandria invented the steam engine two thousand years before the industrial revolution. The engine was considered to have no practical use. Slave labor was so abundant in the ancient world that there was no economic need to industrialize - why build machines when human labor is cheap and plentiful? A steam engine might be a fascinating toy for a conqueror, but it would never become a capital good in the age of Rome. Magic items in ACKS fill the same niche. They are simply too expensive to create to be effective in changing their world.
This does raise the question of "where did the magic items in the dungeons come from"? The answer will, of course, depend on the Judge's campaign setting, but in general ACKS assumes that conditions were different in earlier days. Many campaign settings will assume that magic was stronger "in the old days" (a common trope seen in A Game of Thrones, Lord of the Rings, Conan, The First Law, and more) and thus magic items were cost-effective to make. Another assumption might be that long ago there was an industrio-magic revolution, with mass production techniques. It may be that the ancient items were created by the gods. Or perhaps the items are simply detritus of generations of adventurers. Whatever the case, it's not happening anymore.